Under the proposed definition of "planet," all these bodiesCeres, Charon, 2003 UB313, and perhaps many morewould be granted planethood.

"I think it's good they're grappling with getting a good definition [of the term planet]," said planetary scientist Peter Thomas of Cornell University.

Thomas and colleagues recently announced studies showing that Ceres is smooth and round. Thomas says there are a few other asteroids in the belt that might get upgraded to planetlike status as researchers gather more data.

Flawed Approach?

Despite the scientific basis of the proposed definition, it is stirring up controversy.

Astronomer Mike Brown of CalTech, who led the group that discovered the "tenth planet," opposes the new definitioneven though it would make his discovery officially a planet.

Brown calls the proposed definition "leave no ice ball behind," an approach that's flawed, he said, because it will include far too many objects53 and counting, he figures.

"I'd be sad to miss the chance to have discovered the tenth planet," Brown wrote in a statement. "But I'd get over it."

Astrophysicist Alan Boss of the Carnegie Institution in Washington, D.C., is also critical of the proposed definition.

"It doesn't have the elegance I was hoping for," Boss said. "It looks like it was written by a committee of lawyers rather than scientists."

Boss adds that this is somewhat unavoidable, because of the wide variety of objects in our solar system. But he objects to the broad scope of the definitionand thinks many others will as well.

If there are 50 or more planets, then "being a planet isn't worth what it used to be," Boss said. "Then what's the big deal about being a planet?"